1. Field of the Invention
The present invention relates to a storage disk apparatus for positioning heads over tracks on storage disks and a method of controlling such a storage disk apparatus, and more particularly to a storage disk apparatus having a coarse actuator and microactuators combined with the coarse actuator for displacing heads through minute intervals, and a method of controlling such a storage disk apparatus.
2. Description of the Related Art
There have been demands for increased storage capacities for magnetic disk drives. To meet such demands, it is necessary to reduce the track pitch of magnetic disks. Magnetic heads for use with such magnetic disks need to be positioned accurately over tracks.
Generally, magnetic disk drives employ an actuator comprising a VCM (Voice Coil Motor) for moving magnetic heads across magnetic disks. It is difficult for the actuator which moves rapidly over several thousands to several tens of thousands of tracks on magnetic disks to be positioned over those tracks with an accuracy corresponding to one-tenth to one-ninetieth of the width of a track.
There has been proposed a magnetic disk drive having a two-stage actuator which comprises a coarse actuator in the form of a VCM and a plurality of microactuators for moving heads over minute distances, as disclosed in Japanese laid-open patent publication No. 4-205864, for example.
FIG. 11 of the accompanying drawings shows a structure of the disclosed magnetic disk drive. FIG. 12 of the accompanying drawings shows in block form the disclosed magnetic disk drive. FIG. 13 of the accompanying drawings shows an operation sequence of the disclosed magnetic disk drive.
As shown in FIG. 11, the magnetic disk drive has two magnetic disks 90 rotatable by a spindle motor 92 and an arm 95 actuatable by a VCM 96. The arm 95 supports thereon four microactuators 94-1-94-4 for actuating respective suspensions 93-1-93-4 which support respective heads 91-1-91-4 thereon. The VCM 96 and the arm 95 jointly make up a coarse actuator assembly (second actuator assembly), and the microactuators 94-1-94-4 and the suspensions 93-1-93-4 jointly make up a fine actuator assembly (first actuator assembly). A control circuit 97 positionally controls the VCM 96 and the microactuators 94-1-94-4 based on servo information from the magnetic heads 91-1-91-4.
It has been proposed to use piezoelectric devices having a resonance of a relatively high frequency as the microactuators. However, piezoelectric devices are expensive to manufacture. According to another proposal revealed in Japanese patent application No. 7-315671 (U.S. patent application Ser. No. 728079), electromagnetic microactuators are used as microactuators in magnetic disk drives. While electromagnetic microactuators are relatively inexpensive to manufacture, they have a resonance having a frequency lower than the zero-crossing frequency of open-loop characteristics of a control system therefor.
When the two-stage actuator which includes electromagnetic microactuators is controlled to move the coarse actuator assembly at a high speed in a seek mode for seeking a track on a magnetic disk, for example, the precision actuator assembly tends to be displaced relative to the coarse actuator assembly.
In FIG. 12, the microactuators 94-1-94-4 comprise electromagnetic microactuators, respectively. It is assumed that the microactuators 94-1-94-4 have a sufficiently small mass and identical mechanical characteristics. In FIG. 12, Kv represents an acceleration constant of the VCM 96, Km1-Km4 represent acceleration constants of the respective microactuators 94-1-94-4, Uma1-Uma4 represent currents supplied to the respective microactuators 94-1-94-4, and Y1-Y4 represent displacements of the respective heads 91-1-91-4.
When the VCM 96 generates an acceleration, an acceleration represented by the product of the acceleration generated by the VCM 96 and a gain Kr is applied to the microactuators 94-1-94-4. The microactuators 94-1-94-4 which are not positionally controlled undergo a large relative displacement with respect to the coarse actuator assembly. This phenomenon manifests itself particularly when electromagnetic microactuators having a low-frequency primary resource are employed.
The above phenomenon poses a problem when switching to another head immediately after the seek mode. Specifically, after a certain head has been positioned over a certain track in the seek mode, when the head switches to another head, the microactuator of the other head is vibrated due to the acceleration caused in the seek mode, and undergoes a large relative displacement with respect to the coarse actuator assembly. Consequently, a certain period of time is needed to stabilize the vibration and the relative displacement.
According to the disclosure of Japanese laidopen patent publication No. 4-205864, the above problem is solved by positionally controlling the microactuators based on positional signals (servo signals) detected by the respective heads.
Such a control process with N microactuators will be described below with reference to FIG. 13. Successive steps of the control process are represented by numerals with a prefix S.
(S1) A pointer i is initialized to "1" for each sample.
(S2) Control checks whether or not the pointer i is equal to or greater than "N+1".
(S3) If the pointer i is not equal to or greater than "N+1", then the position of the ith head is read from the servo signal, and a drive current value for the ith microactuator is calculated.
(S4) The ith microactuator is energized with the calculated drive current value. Then, the pointer i is updated to "i+1", and control returns to the step S2.
(S5) If the pointer i is equal to or greater than "N+1", then a drive current value for the coarse actuator assembly is calculated, using the position of the head to be controlled. Thereafter, the coarse actuator assembly is energized, after which the operation sequence comes to an end.
Heretofore, as described above, the servo signal of each of the heads is read for each sample, and the position of each of the heads is detected from the servo signal. Then, a drive current value for each of the microactuators is calculated, and each of the microactuators is energized. In this manner, each of the microactuators is prevented from being vibrated and relatively displaced in the seek mode.
However, the conventional control process has been disadvantageous for the following reasons:
According to an embeded servo system, only one position detecting circuit is employed. One of a plurality of heads is selected, and a positional signal read by the selected head is supplied to the position detecting circuit. According to the conventional magnetic disk drive, however, as many position detecting circuits as the number of microactuators are required to detect the positions of the respective microactuators. Therefore, the conventional magnetic disk drive is expensive to manufacture.
Furthermore, drive current values for all the microactuators need to be calculated for each sample. Consequently, the conventional magnetic disk drive requires a high-speed processor, and hence is expensive to manufacture.